1. Field of the Invention
The present invention relates to a hermetically sealed battery which is equipped with an electrode group, wherein a positive electrode and negative electrode are disposed in such a way that they are wound and separated from each other by a separator, an outside can for storing the electrode group with an opening used as a terminal for one of the positive and negative electrodes, and an opening to serve as a sealing unit with a pressure valve, the sealing unit functioning as a terminal for the other electrode to close the opening of the outside can hermetically by means of an insulation gasket. More specifically, the present invention relates to a hermetically sealed battery, which includes a Positive Temperature Coefficient (PTC) element in a opening sealing unit.
2. Description of the Related Art
Generally, a hermetically sealed battery, such as a nickel-hydrogen battery, or a nickel-cadmium battery or the like, is operated at an applied voltage of about 1.2 V in the discharge mode. As a result, a battery of the so-called AA or AAA size can be used as a compatible battery for an AA or AAA size manganese dry cell or alkali primary battery (alkali dry cell). When a manganese dry cell or alkali dry cell is used in an erroneous manner, e.g., when the external circuit on the load side is short circuited or when the battery is inversely mounted in an inverse orientation, a large output current does not flow because of inferior output property. However, such a hermitically sealed battery as described above provides a large current even when a short circuit occurs, because the battery is capable of supplying a large discharge current, and consequently a burnout often takes place due to overheating and/or heavy current flow.
To prevent the occurrence of such burnout due to overheating and/or heavy current flow, a lithium ion battery normally includes either a PTC element for suppressing the flow of a large current with an increased resistance resulting from the increase in temperature, or a breaker for interrupting such flow. For instance, Japanese Patent Laid-Open Publication No. H02(1990)-207450 has proposed that the PTC element be disposed on the bottom of the outside can, where the bottom is used as a terminal for the positive electrode. However, if the PTC element is mounted upon a location other than the opening sealing unit, the accuracy of monitoring the temperature inside the valve chamber is reduced. In view thereof, Japanese Patent Publication No. 3143176 has proposed that the PTC element be disposed in an opening close unit having a valve body.
The structure of such a opening sealing unit having a PTC element is shown in FIG. 3. The opening sealing unit comprises a stainless steel positive electrode cap 31 made in the shape of a cap and a bottom plate 34 made in the shape of a dish. The positive electrode cap 31 is comprised of a convex portion 32 swelling outwardly from the battery and a flange portion 33 forming the bottom of the convex portion 32 in the shape of a flat plate, wherein there is a plurality of gas discharging holes 32a at the corners of the convex portion 32. On the other hand, the bottom plate 34 has a concave portion 35 swelling inwardly towards the battery and a flange portion 36 forming the bottom of the concave portion 35 in the form of a flat plate, wherein there is a plurality of gas discharging holes 35a at the corners of the concave portion 35.
An electric power guiding plate 37 capable of deforming at a gas pressure greater than a predetermined value inside the battery is made to reside at the space between the positive electrode cap 31 and bottom plate 34. The electric power guiding plate 37 consists of a concave portion 37a and a flange portion 37b, both made of aluminum foil. The concave portion 37a is disposed in such manner that the lowest part thereof is in contact with the upper surface of the concave portion 35 at the bottom plate 34. The flange portion 37b is clamped between the flange portion 33 of the positive electrode cap 31 and the flange portion 36 at the bottom plate 34. In this case, the positive electrode cap 31 and the bottom plate 34 are sealed liquid tight by means of a polypropylene (PP) insulation gasket 39 for the opening sealing unit.
Meanwhile, a PTC (Positive Temperature Coefficient) element 38 is disposed on the upper part of the flange portion 37b. When an overcurrent flows in the battery, abnormal heat is generated, increasing the resistance of the PTC element 38, which in turn causes such overcurrent to be suppressed. Furthermore, when the increase of gas pressure inside the battery is greater than a predetermined value, the concave portion 37a of the electric power guiding plate 37 is deformed, causing the electric power guiding plate 37 and the concave portion 35 of the bottom plate 34 to lose contact with each other, thereby allowing the overcurrent or the short circuit current to be interrupted.
However, as shown in FIG. 4, in the case of an alkali secondary battery using a opening sealing unit without a PTC element, the opening sealing unit 40 is designed in such a way that the thickness at the outer periphery (the part caulked to an outside can 47 by an insulation gasket 46) is reduced, allowing for an increase in the volume of the battery, thereby increasing the capacity of electric charge to be discharged. That is, the opening sealing unit 40 consists of a cap-shaped positive electrode cap 41 and a disk-shaped bottom plate 42 both made of nickel-plated steel plates welded to each other.
Thereafter, a gas discharging opening 42a is constituted at the center of the bottom plate 42, and a flange portion 42b is formed at the outer periphery of the bottom plate 42. A pressure valve consisting of a valve plate 43 and a spring 45 is disposed at the space between the positive electrode cap 41 and the bottom plate 42. In this case, a nickel-plated steel plate 44 is interposed between the valve plate 43 and the spring 45. Moreover, the flange portion 42b of the bottom plate 42 is clamped by a polypropylene (PP) insulation gasket 46, which is caulked to and rests upon a deep-drawn portion 47a constituted on the upper part of the outside can 47.
However, as shown in FIG. 3, a laminate structure consisting of the flange portion 36, the insulation gasket 39, the PTC element 38 in the bottom plate 34, the flange portion 33 in the positive electrode cap 31, and the caulked portion 36a in the bottom plate 34 is formed when the opening sealing unit is equipped with the PTC element. In this manner, the outer periphery of the opening sealing unit (the portion caulked to the outside can through the insulation gasket) becomes thick in consistency, thereby reducing the space for storing the electrode group and thus it must be equipped with a PTC element at the expense of the charge capacity of the battery.
As distinguished from the lithium battery, the alkali secondary battery is equipped with a non-fracture type resilient valve for discharging gas. Therefore, when a functional component such as the PTC element is disposed inside the opening sealing unit, alkali mist normally discharged in very small amounts causes the PTC element to deteriorate, thereby making it difficult to mount the PTC element upon the opening sealing unit in a unified form.